Heterogeneous forms of bovine and mouse TSH show differences in their bioactivity (B/I) ratio. Examination of the interaction between certain forms of mouse tumor or bovine pituitary TSH with standard bTSH revealed that those forms with low B/I act as competitive antagonists to more bioactive forms. Lectin-binding studies suggest that there are differences in carbohydrate content between the TSH with high biologic activity and that with lower activity.